Filter aids are taken to mean additives which are used in solid-liquid separation processes in order, by formation of a porous precoat layer on the actual filter medium and/or by incorporation into the filter cake framework, to ensure separation of the solids with simultaneously sufficient permeability of the resultant filter cake.
As filter aids, use is made of both inorganic substances such as, for example, kieselguhr or aluminum oxides, or else synthetic polymers. Which individual filter aids are used depends on the field of application. In the filtration of beer, kieselguhr is one of the filter aids principally used.
For economic reasons it is advantageous when the filter aid can be regenerated. A regeneration over a plurality of cycles is particularly advantageous.
WO 02/32544 describes coextrudates of polystyrene and water−insoluble polyvinylpyrrolidone and use thereof as regenerable filter aids, but the regenerability is only mentioned quite in general.
WO 03/084639 describes coextrudates of thermoplastic polymers, apart from polystyrene, and water-insoluble polyvinylpyrrolidone and use thereof as regenerable filter aids, but the regenerability is only mentioned quite in general.
WO 92/11085 describes filter aids based on agglomerates of crosslinked polyvinylpyrrolidone and fibrous thermoplastic polymers such as, for example, polyethylenes or polyamides, and use thereof as filter aids. It is noted quite in general terms that the filter aids are regenerable.
EP-A 611249 describes a process for regenerating a filter aid by addition of enzymes. Only the treatment of kieselguhr is described specifically.
EP-A 253 233 describes the regeneration of kieselguhr by means of sodium hydroxide solution.
DE 19625481 describes the regeneration of kieselguhr in the presence of a mixture of nonionic surfactants.
DE 196 52 499 describes the regeneration of filter aids of kieselguhr, the filter aids first being treated with enzyme solutions and thereafter first with weak alkalis, then with weak acids, and finally with an oxidizing agent.
WO 03/008067 describes the regeneration of filter aids, first flushing with hydroxide solution on the intact filter cake and subsequently, also on the intact filter cake, a neutralization of the pH by flushing with acid being performed. WO 96/35497 discloses regeneration of filter aids made of synthetic polymers by washing with sodium hydroxide solution and washing with an enzymatic compound, the treatment being performed in situ in the filter unit on the intact filter cake. What is described is especially the regeneration of a polyamide filter aid.
However, it has been found that the previously known processes for the filter aids to be treated according to the invention do not give satisfactory results. Using these processes, satisfactory regeneration of the coextrudate over a plurality of cycles is not possible, and the good filtration properties of the fresh filter aid with respect to pressure rise during filtration and also the filter and wash resistances in the regenerated material are not retained.
Customarily, filter resistances and washing resistances which exceed a certain value, depending on technical field of application and filter aid, are not considered acceptable in practice, since otherwise they provoke excessively high rates of pressure increase during the beer filtration, which results in uneconomically short filtration times. In the case of beer filtration, for example the filter resistances and washing resistances for the filter aid to be treated according to the invention should as far as possible not exceed a value of 10×1012 mPas/m2.
Filter resistance is the product of fluid viscosity and resistance to flow during the buildup of filter cake, and washing resistance is the product of fluid viscosity and resistance to flow on flow through the filter cake which is already built up. Determination of the corresponding measured values is known to those skilled in the art and is extensively described in VDI Guideline 2762.